The role of secondary heart field in cardiac development

Although de la Cruz and colleagues showed as early as 1977 that the outflow tract was added after the heart tube formed, the source of these secondarily added cells was not identified for nearly 25 years. In 2001, three pivotal publications described a secondary or anterior heart field that contributed to the developing outflow tract. This review details the history of the heart field, the discovery and continuing elucidation of the secondarily adding myocardial cells, and how the different populations identified in 2001 are related to the more recent lineage tracing studies that defined the first and second myocardial heart fields/lineages. Much recent work has focused on secondary heart field progenitors that give rise to the myocardium and smooth muscle at the definitive arterial pole. These progenitors are the last to be added to the arterial pole and are particularly susceptible to abnormal development, leading to conotruncal malformations in children. The major signaling pathways (Wnt, BMP, FGF8, Notch, and Shh) that control various aspects of secondary heart field progenitor behavior are discussed.     

BMP signaling modulates hedgehog-induced secondary heart field proliferation

Sonic hedgehog signaling in the secondary heart field has a clear role in cardiac arterial pole development. In the absence of hedgehog signaling, proliferation is reduced in secondary heart field progenitors, and embryos predominantly develop pulmonary atresia. While it is expected that proliferation in the secondary heart field would be increased with elevated hedgehog signaling, this idea has never been tested. We hypothesized that up-regulating hedgehog signaling would increase secondary heart field proliferation, which would lead to arterial pole defects. In culture, secondary heart field explants proliferated up to 6-fold more in response to the hedgehog signaling agonist SAG, while myocardial differentiation and migration were unaffected. Treatment of chick embryos with SAG at HH14, just before the peak in secondary heart field proliferation, resulted unexpectedly in stenosis of both the aortic and pulmonary outlets. We examined proliferation in the secondary heart field and found that SAG-treated embryos exhibited a much milder increase in proliferation than was indicated by the in vitro experiments. To determine the source of other signaling factors that could modulate increased hedgehog signaling, we co-cultured secondary heart field explants with isolated pharyngeal endoderm or outflow tract and found that outflow tract co-cultures prevented SAG-induced proliferation. BMP2 is made and secreted by the outflow tract myocardium. To determine whether BMP signaling could prevent SAG-induced proliferation, we treated explants with SAG and BMP2 and found that BMP2 inhibited SAG-induced proliferation. In vivo, SAG-treated embryos showed up-regulated BMP2 expression and signaling. Together, these results indicate that BMP signaling from the outflow tract modulates hedgehog-induced proliferation in the secondary heart field.     

冻存对自发性高血压大鼠精子顶体功能的影响

应用金霉素(CTC)荧光检测、精子穿卵试验和项体酶β-D-半乳糖苷酶活力测定等方法检测冻存对大鼠精子顶体的影响。CTC荧光反应显示,与未冻存精子比较,冻存后精子顶体反应的类型发生改变,AR型(发生了顶体反应)精子比例明显下降,冻存前为68.6％,冻存后为13.4.％,但是获能精子的比例未发生明显变化(92.6％对90.8％)。与未冻存的精子相比,冻存组精子的穿卵试验的受精指数下降(23.4±7.02对10.2±3.95),而且冻存后精子顶体酶——β-D-半乳糖苷酶活力明显下降(9.39±1.98对4.50±1.40)。结果表明冻存后精子的顶体功能受到较明显的破坏。实验结果为今后针对性地改进大鼠精子的冻存方法,改善大鼠精子的冻存保护剂提供实验和机制上的依据。     

Nud1p links astral microtubule organization and the control of exit from mitosis

The budding yeast spindle pole body (SPB) not only organizes the astral and nuclear microtubules but is also associated with a number of cell-cycle regulators that control mitotic exit. Here, we describe that the core SPB component Nud1p is a key protein that functions in both processes. The astral microtubule organizing function of Nud1p is mediated by its interaction with the gamma-tubulin complex binding protein Spc72p. This function of Nud1p is distinct from its role in cell-cycle control: Nud1p binds the spindle checkpoint control proteins Bfa1p and Bub2p to the SPB, and is part of the mitotic exit network (MEN) in which it functions upstream of CDC15 but downstream of LTE1. In conditional lethal nud1-2 cells, the MEN component Tem1p, a GTPase, is mislocalized, whereas the kinase Cdc15p is still associated with the SPB. Thus, in nud1-2 cells the failure of Tem1p to interact with Cdc15p at the SPB probably prevents mitotic exit.     

Mechanisms of microtubule-based kinetochore positioning in the yeast metaphase spindle

It has been hypothesized that spatial gradients in kMT dynamic instability facilitate mitotic spindle formation and chromosome movement. To test this hypothesis requires the analysis of kMT dynamics, which have not been resolved at the single kMT level in living cells. The budding yeast spindle offers an attractive system in which to study kMT dynamics because, in contrast to animal cells, there is only one kMT per kinetochore. To visualize metaphase kMT plus-end dynamics in yeast, a strain containing a green fluorescent protein fusion to the kinetochore protein, Cse4, was imaged by fluorescence microscopy. Although individual kinetochores were not resolvable, we found that models of kMT dynamics could be evaluated by simulating the stochastic kMT dynamics and then simulating the fluorescence imaging of kMT plus-end-associated kinetochores. Statistical comparison of model-predicted images to experimentally observed images demonstrated that a pure dynamic instability model for kMT dynamics in the yeast metaphase spindle was unacceptable. However, when a temporally stable spatial gradient in the catastrophe or rescue frequency was added to the model, there was reasonable agreement between the model and the experiment. These results provide the first evidence of temporally stable spatial gradients of kMT catastrophe and/or rescue frequency in living cells.     

Nuclear localization of beta-catenin in vegetal pole cells during early embryogenesis of the starfish Asterina pectinifera

Recently, beta-catenin has been reported to control the expression of morphogenetic genes through the Wnt signaling pathway in invertebrate embryogenesis. In this study, the distribution pattern of beta-catenin during starfish embryogenesis was investigated using immunohistochemistry. In 16-cell stage embryos, beta-catenin began to accumulate in some nuclei at the vegetal pole. During the early cleavage stage, the cells expressing nuclear beta-catenin increased in number in the vegetal pole region of the embryos, and the beta-catenin signal increased in intensity in each nucleus. At the blastula stage, signal for beta-catenin was also found in the cytoplasm of the cells with nuclear beta-catenin. At the vegetal plate stage, almost all vegetal plate cells expressed beta-catenin in both the nucleus and cytoplasm. When the embryos developed to early gastrulae, cells with nuclear beta-catenin were restricted to the archenteron tip, and the signal gradually faded in later stages. The localization and temporal change of beta-catenin expression suggests that beta-catenin has a pivotal role in archenteron formation in starfish embryos.     

Prospore membrane formation: how budding yeast gets shaped in meiosis

During meiosis in Saccharomyces cerevisiae four daughter cells, called spores, are generated within the boundaries of the mother cell. This cell differentiation process requires de novo synthesis of prospore membranes (PSMs), which are the precursors of the spore plasma membranes. Assembly of these membranes is initiated at the spindle pole bodies (SPBs) during meiosis II. At this stage of the cell cycle, 4 SPBs are present. Two different meiosis-specific structures are known to be required for PSM formation. At the SPBs, specialized attachments, called the meiotic plaques, provide the required functionality necessary for the recruitment and assembly of the membranes. During subsequent membrane elongation, a second structure becomes important. This proteinaceous assembly forms a coat, called the leading edge protein coat (LEP coat), which covers the boundaries of the membranes. Assembly of the coat occurs at sites next to the SPBs, whereas its disassembly is concomitant to the closure of the membranes. This mini review discusses our current understanding of how the meiotic plaque and the LEP coat might function during biogenesis of the prospore membrane.     

Mps3p is a novel component of the yeast spindle pole body that interacts with the yeast centrin homologue Cdc31p

Accurate duplication of the Saccharomyces cerevisiae spindle pole body (SPB) is required for formation of a bipolar mitotic spindle. We identified mutants in SPB assembly by screening a temperature-sensitive collection of yeast for defects in SPB incorporation of a fluorescently marked integral SPB component, Spc42p. One SPB assembly mutant contained a mutation in a previously uncharacterized open reading frame that we call MPS3 (for monopolar spindle). mps3-1 mutants arrest in mitosis with monopolar spindles at the nonpermissive temperature, suggesting a defect in SPB duplication. Execution point experiments revealed that MPS3 function is required for the first step of SPB duplication in G1. Like cells containing mutations in two other genes required for this step of SPB duplication (CDC31 and KAR1), mps3-1 mutants arrest with a single unduplicated SPB that lacks an associated half-bridge. MPS3 encodes an essential integral membrane protein that localizes to the SPB half-bridge. Genetic interactions between MPS3 and CDC31 and binding of Cdc31p to Mps3p in vitro, as well as the fact that Cdc31p localization to the SPB is partially dependent on Mps3p function, suggest that one function for Mps3p during SPB duplication is to recruit Cdc31p, the yeast centrin homologue, to the half-bridge.     

Partial purification and characterization of an alkaline proteinase from the rabbit testis

A proteolytic enzyme capable of cleaving intact proteins and synthetic substrates α?N?benzoyl?DL?arginine β?naphthylamide (Bz-Arg-NNap), α-N-benzoyl-L-arginine p-nitroanilde (Bz-Arg-NPhNO₂), and α-N-benzoyl-L-arginine ethyl ester (Bz-Arg-OEt) was purified 92– fold from the rabbit testes. The enzyme exhibited optimal activity at pH 9.0 and 50°C. The polyacrylamide gel electrophoresis and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of the purified enzyme demonstrated multiple forms; the major band in the SDS-polyacrylamide gel electrophoresis corresponded to a M_t 48,000. The same value was established by the gel filtration over Sephadex G-75. The rabbit testicular alkaline proteinase (TAP) resembled acrosin in the hydrolysis of Bz-Arg-OEt. However, CaCl₂, a potential stimulator of acrosin activity, inhibited the alkaline proteinase. The strong inhibitors of acrosin, eg pheny methyl sulphonyl fluoride (PMSF), tosyl lysine chloromethyl ketone (TLCK), and benzamidine did not inhibit the alkaline proteinase. TAP was activated by an acrosin inhibitor isolated from the rabbit testes. Since 0.5 M KCl was necessary for complete extraction of the enzyme and the bulk of the activity was present in 9,000g pellet of the testicular homogenate. The alkaline proteinase appeared to be associated with the membranous structures.     

Saccharomyces cerevisiae Ndc1p Is a Shared Component of Nuclear Pore Complexes and Spindle Pole Bodies

We report a novel connection between nuclear pore complexes (NPCs) and spindle pole bodies (SPBs) revealed by our studies of the Saccharomyces cerevisiae NDC1 gene. Although both NPCs and SPBs are embedded in the nuclear envelope (NE) in yeast, their known functions are quite distinct. Previous work demonstrated that NDC1 function is required for proper SPB duplication (Winey, M., M.A. Hoyt, C. Chan, L. Goetsch, D. Botstein, and B. Byers. 1993. J. Cell Biol. 122:743–751). Here, we show that Ndc1p is a membrane protein of the NE that localizes to both NPCs and SPBs. Indirect immunofluorescence microscopy shows that Ndc1p displays punctate, nuclear peripheral localization that colocalizes with a known NPC component, Nup49p. Additionally, distinct spots of Ndc1p localization colocalize with a known SPB component, Spc42p. Immunoelectron microscopy shows that Ndc1p localizes to the regions of NPCs and SPBs that interact with the NE. The NPCs in ndc1-1 mutant cells appear to function normally at the nonpermissive temperature. Finally, we have found that a deletion of POM152, which encodes an abundant but nonessential nucleoporin, suppresses the SPB duplication defect associated with a mutation in the NDC1 gene. We show that Ndc1p is a shared component of NPCs and SPBs and propose a shared function in the assembly of these organelles into the NE.